System and method of dehydrogenative coupling

ABSTRACT

Dehydrogenative coupling can be achieved in nearly quantitative conversions and yields using a membrane reactor.

CLAIM OF PRIORITY

This application claims the benefit of prior U.S. Provisional Application No. 61/643,258, filed on May 5, 2012, which is incorporated by reference in its entirety.

TECHNICAL FIELD

This invention relates to methods and systems for dehydrogenative coupling.

BACKGROUND

A series of homogeneous and heterogeneous catalysts have been developed to catalyze a variety of dehydrogenation reactions, however, the use of the catalysts successfully in industry can require complicated processes in order to optimize production of certain products.

SUMMARY

A dehydrogenative coupling product can be produced by coupling a substrate to form a dehydrogenative coupling product and hydrogen and separating hydrogen from the dehydrogenative coupling product using a selectively permeable membrane. For example, green production of ethyl acetate and hydrogen from ethanol is achieved in nearly quantitative conversions and yields by utilizing a catalyst, such as dearomatized PNN—Ru(II) catalysts, in a membrane reactor, such as an ultrathin Pd—Ag alloy/ceramic membrane reactor. The coupling can be achieved without the need of acid or base promoters and hydrogen acceptors.

In one aspect, a method of producing a dehydrogenative coupling product can include exposing a substrate to a catalyst in a reaction zone of a reactor, coupling the substrate to form the dehydrogenative coupling product and hydrogen, and separating the hydrogen from the dehydrogenative coupling product using a selectively permeable membrane and passing the hydrogen to a gas release zone of the reactor.

In another aspect, a system for producing a dehydrogenative coupling product can include a reactor including a reaction zone and a gas release zone separated by a selectively permeable membrane, wherein the selectively permeable membrane permits hydrogen to pass through the membrane and substantially blocks a substrate and its dehydrogenative coupling product from passing through the membrane. The gas release zone can be configured to transport hydrogen out of the reactor. The reaction zone can surround the gas release zone. The selectively permeable membrane can be cylindrical.

In certain embodiments, the selectively permeable membrane includes a metal membrane on a solid support. The metal membrane can include palladium. The solid support can include a silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, magnesium oxide, cerium oxide, zinc oxide, molybdenum oxide, iron oxide, nickel oxide, cobalt oxide, graphite, or stainless steel.

In certain embodiments, the substrate can be a low boiling point alcohol. The catalyst can be a homogeneous catalyst. The dehydrogenative coupling product can be an ester. The substrate and the catalyst can be substantially free of solvent. In preferred embodiments, conversion of the substrate to the dehydrogenative coupling product is substantially quantitative.

Other features or advantages will be apparent from the following detailed description of several embodiments, and also from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram depicting a system for dehydrogenative coupling.

FIG. 2 is a graph depicting pressure dependence of H₂ flux of a Pd—Ag membrane at 150-350 K before reaction.

FIG. 3 is a graph depicting temperature dependence of H₂ flux of Pd—Ag membrane with ΔP_(H2)=100 kPa.

FIG. 4 are two photographs depicting scanning electron microscopy top view (a) and cross-section view of Pd—Ag membrane after reaction.

FIG. 5 is a schematic diagram depicting a system for dehydrogenative coupling.

FIG. 6 is a graph depicting the hydrogen liberation rate and the total pressure of ethanol dehydrogenative coupling reaction at different reaction times at 436K.

FIG. 7 is a graph depicting hydrogen fluxes of ethanol dehydrogenative coupling depending on the reaction temperature.

FIG. 8 is a graph depicting hydrogen fluxes of ethanol dehydrogenative coupling depending on the loading of catalyst at 160° C. over 24 hours.

DETAILED DESCRIPTION

A series of homogeneous and heterogeneous catalysts have been developed to catalyze a variety of dehydrogenation reactions such as dehydrogenative coupling of alcohols to esters, dehydrogenative coupling of alcohols and amines to amides and urea, dehydrogenation of alkanes to alkenes, dehydrogenative coupling of alkanes to higher homologs, etc. Examples are shown below.

However, two major issues can limit the commercial development of the dehydrogenative processes. First, since the standard reaction temperature is in the range of 110-300° C., substrates with low boiling points (<100° C.) can be difficult to handle. Also, if the reactions are conducted in a traditional closed system (e.g. in a Parr reactor), the low yields were experienced due to the low conversion as the system reaches equilibrium.

The system and method described here is a process to design and utilize a metal membrane reactor such that the above mentioned reactions can be achieved in high yields and the valuable hydrogen byproduct can be separated and stored, or used as the alternative energy source to low down the energy consumption. The efficacy of this design has been fully examined with the following setup on dehydrogenative coupling of ethanol to ethyl acetate. In certain circumstances, over 99% conversion and yield can be achieved.

By way of example, industrial production of ethyl acetate is a highly energy intensive process which requires the reaction of ethanol and acetic acid in the presence of a strong acid (H₂SO₄) at 70% conversion followed by distillation. In the process described herein, dehydrogenative coupling of ethanol (b.p. 78° C.) can be achieved at 160-200° C. with quantitative conversion to ethyl acetate and two equivalent of hydrogen can be separated. This more economical process provides higher conversion and higher yield compared to current art. In general, membranes can now be used to design a reactor for dehydrogenation reactions of low boiling chemicals (<120° C.).

Ethyl acetate (EA) is an important industrial product employed in a wide range of applications from solvent to intermediates. See Ref. 1. In the industrial processes, EA is mainly produced via Fischer esterification of ethanol (EtOH) and acetic acid, and via Tishchenko reaction of acetaldehyde or addition of acetic acid to ethylene (Avada) on a smaller scale. See Refs. 2-5. These processes, in addition to the use of corrosive and/or toxic reagents/substrates, require energy-intensive distillation operations. Direct dehydrogenative coupling of EtOH to EA with the liberation of H₂ is therefore an attractive approach as it offers a simple, noncorrosive, relatively nontoxic and economic process. It only consumes EtOH, an inexpensive renewable raw material, as the feedstock, but provides EA and valuable hydrogen as the products. Many heterogeneous catalysts have been examined for this goal, yet most of them suffer from the low conversion rates and/or selectivity. See Refs. 6-10. As EtOH forms an azeotrope with EA at composition of 69.2%, separation of EA from the resulting EtOH mixtures is very challenging and energy-consuming. On the other hand, homogeneous catalysts have the potentials to achieve high conversion and selectivity, and in fact, a series of Ru pincer complexes have showed promising catalytic activities in the dehydrogenative coupling of primary alcohols to esters in high yields at 110-160° C. See refs. 11-12. However, extension of these reactions to alcohols with lower boiling points, such as ethanol and 1-propanol, still represents an issue due to the high reaction temperature needed.

PNN—Ru(II) complexes 1 and 3 show comparable catalytic activities towards dehydrogenation of primary alcohols to corresponding esters to that of Milstein catalyst 2 (Table 1). Consistent with the literature results, when EtOH was employed, no reaction was observed after heating at reflux in the presence of 1 for 24 h. See Ref. 12. However, 69-75% conversions were achieved by carrying out the reactions in a pressure tube with 1-3, indicating that the pincer-Ru(II) catalysts indeed also work effectively for small alcohols when the reaction can be conducted at elevated temperatures. Since a closed system will be required to reach such a condition, we rationalize that the moderate conversions were due to the equilibrium reached: as the total pressure builds up because of the formation of H₂, the chemical equilibrium is backshifted. 2EtOH→EA+2H₂ΔrH_(298K) ^(θ)=26 kJmol⁻¹, ΔrS _(298K) ^(θ)=52.2 Jmol⁻¹ , ΔrG _(298K) ^(θ)=7.8 kJmol⁻¹

TABLE 1 Esterification of primary alcohol with PNN-Ru(II) complexes.^(a)

1

2

3

Cat. Loading Entry R Cat. (mol %) T (° C.) Conv. (%)^(b) 1  CH₃(CH₂)₄ ⁻ 1 0.1 160 98 2  CH₃(CH₂)₄ ⁻ 2 0.1 157  91.5^(c) 3  Me 1 0.5  78  0 4^(d) Me 1 0.5 160 75 5^(d) Me 2 0.5 160 75 6^(d) Me 3 0.5 160 69 ^(a)Alcohol (5 mmol), suitable amount of ruthenium complex in a Schlenk flask equipped with a cooling finger are heated at the stated temperature under argon flow for 24 h; ^(b)Determined by ¹H NMR (>99% selectivity for EA formation); ^(c)Ref. 11; ^(d)the reaction were performed in 25 mL sealed tube under argon atmosphere.

From its thermodynamics, the formation of EA and H₂ is preferred at high temperature and under low pressure. Therefore, if the generated H₂ can be selectively in situ separated from the reaction mixtures, one will be able to drive the reaction to completion. In this regard, supported Pd-based composite membranes are the most applicable candidates for the selective hydrogen removal owing to their high and exclusive hydrogen permeability, high thermal stability, moderate chemical resistance and mechanical strength. Beller and co-workers just reported that up to 81% yield could be achieved for the dehydrogenation of EtOH to EA with the use of a more reactive Ru catalyst at 90° C. Considering the extreme difficulty in the separation of EtOH and EA, higher conversions and yields will be practically desirable. The design employs a defect-free ultrathin Pd—Ag/ceramic membrane reactor to overcome the above-mentioned issues to achieve quantitative conversion of EtOH to EA and H₂ without the use of solvent, acid or base promoters, or additional H₂ acceptors.

A method of producing a dehydrogenative coupling product includes catalytically coupling a subtrate to form a dehydrogenative coupling product and hydrogen gas. The substrate can be an alcohol, in particular, a C1-C16 primary alcohol, a C1-C8 primary alcohol, or a C1-C5 primary alcohol. Preferably, the alcohol can be a low boiling point alcohol having a boiling point of less than 250° C., less than 200° C., less than 150° C., or less than 100° C. Green and renewable alcohols, such as ethanol, propanol, butanol, etc., can be particularly attractive for use in this method. The dehydrogenative coupling product can be an alkyl ester of the alcohol, such as, for example, methyl formate, ethyl acetate, propyl propanoate, butyl butanoate, or pentyl pentanoate. In certain circumstances, the substrate can be a mixture of two or more alcohols. In other circumstance, the substrate and the catalyst can be substantially free of solvent, in which case the substrate serves as the solvent.

The catalyst can be a transition metal catalyst, such as a transition metal complex of, for example, ruthenium, nickel, iron, copper, cobalt, palladium, or platinum. The catalyst can be a homogeneous catalyst, such as a ruthenium(II) catalyst described in Nielsen, et al. Angew. Chem. Int. Ed. 2012, 51, or U.S. Provisional Patent Application No. 61/499,028, each of which is incorporated by reference in its entirety.

Importantly, the hydrogen can be separated from the dehydrogenative coupling product using a selectively permeable membrane. The hydrogen can then be passed into a gas release zone of the reactor and removed from the system. This process can assist in shifting the equilibrium of the coupling reaction to the coupled product, and can lead to nearly quantitative or quantitative conversion to product (e.g., 90%, 95%, 98%, 99%, 99.5% or higher completion). As a result, if highly pure starting materials are used, highly pure products are obtained. In certain circumstances, the products merely need to be removed from the catalyst by distillation, evaporation or filtration.

The selectively permeable membrane can include a metal membrane on a solid support. The membrane can be formed on the support or placed on the support. The metal membrane can include palladium. The selectively permeable membrane membrane permits hydrogen to pass through the membrane and substantially blocks a substrate and its dehydrogenative coupling product from passing through the membrane. In certain circumstances, the selectively permeable membrane allows hydrogen to pass, but not ethanol or ethyl acetate. Suitable membranes can be prepared, for example, by electroless plating. See Refs. 20-21.

The solid support can be a porous ceramic, for example, metal oxides, refractory oxides and molecular sieves, in particular from silicon oxides, aluminum oxides, zeolites, clays, titanium oxide, cerium oxide, magnesium oxide, niobium oxide, zinc oxide, molybdenum oxide, iron oxide, cobalt oxide, tantalum oxide or zirconium oxide, or a porous stainless steel structure, such as a metal frit, or mixtures thereof.

The method can be carried out at a selected temperature of about 300° C. or less, about 250° C. or less, about 200° C. or less, or about 180° C. or less. The method can be carried out at the pressure of the autogeneous saturation pressure of the liquid products between about 0.5 to 20 bar, or between about 1 and 10 bar.

Referring to FIG. 1, a system for producing a dehydrogenative coupling product can include a reactor 5 having a reaction zone 10 and a gas release zone 20 separated by a selectively permeable membrane 30. Reaction zone 10 can surround gas release zone 20. The gas release zone can be configured to transport hydrogen out of the reactor through port 40. The selectively permeable membrane can be cylindrical, columnar or have another long shape having a hollow interior region.

EXAMPLES

The following examples illustrate the systems and methods described herein, but are not meant to be limiting.

Preparation of Pd—Ag Membrane

35 mm long asymmetric Al₂O₃ membrane tubes were used as support, which has a mean surface pore size of 100 nm and inner and outer diameters of 7 and 10 mm (supplied by Inopor). After cleaning in ethanol the tubes were rinsed with deionized water, then soaked in 4% aqueous KOH solution and finally rinsed with DI water. Then the support was activated sequentially by seeding aqueous solutions (PdSO₄ 10 mmol/L, EDTANa₂ 1 mmol/L) at 60° C. and N₂H₄ dilute solution (20 mmol/L) at room temperature several times until the surface turned uniform dark grey. The Ag-controlled co-plating method was employed to prepare the Pd—Ag membrane. In a typical process, 60 mL Pd bath was loaded into a glass container and 15 mL Ag bath in a syringe. The Pd bath contained 3.9 mmol/L PdCl₂, 5 mol/L NH₄OH and 0.12 mol/L Na₂EDTA dissolved in deionized water, and the Ag bath contains 5.1 mmol/L AgNO₃, 5 mol/L NH₄OH, and 0.12 mol/L Na₂EDTA mixed in deionized water. The activated support was vertically placed inside the Pd bath. Then, 0.45 ml N₂H₄ solution (1 mol/L) was added to the Pd bath and well mixed. Immediately, the Ag bath started to feed into the bottom of the Pd bath with the programming feeding rate (0.1 mL/min for 45 min and progressively decreased by 0.01 mL/min every 30 min interval). To achieve a defect-free membrane, the plating process above was conducted twice. After the first plating, the membrane was washed, dried and then annealed 10 h at 550° C. in H₂. Then the membrane was seeded again before the second co-plating, resulting in a 5 μm thick Pd₇₅Ag₂₅ membrane. The final Pd—Ag membrane was heat-treated at 500° C. in H₂ for 36 h before used for reaction.

Characterizations of Pd—Ag Membrane

The membranes were characterized by scanning electron microscopy (SEM, Quanta 600G, FEI) and energy-dispersive X-ray spectroscopy (EDX). The crystal structure and metal content of the alloy membranes were also determined by X-ray diffraction (XRD, Brokes) using Cu Kα radiation with voltage set at 40 kV and current at 40 mA. For example, FIG. 2 shows the pressure dependence of H₂ flux of Pd—Ag membrane at 150-350 K before reaction. FIG. 3 shows the temperature dependence of H₂ flux of Pd—Ag membrane with ΔP_(H2)=100 kPa.

Membrane Reaction

The tubular alloy membrane was mounted in a custom-designed reaction cell, as shown in FIG. 5, with graphite gasket O-rings at both ends to reactor shell, which was placed inside a vertical furnace with programmable temperature controller. The reaction chamber had an effective membrane surface area of approx. 6 cm² and the volume of approx. 2 mL between the membranes and the shell of the cell, where the solution of EtOH and catalysts were contained. The reaction temperature was fixed on 160, 180 and 200° C., respectively, for different reactions. The membrane reactor was placed inside furnace after reaching the target temperature. 1 mL ethanol/PNN catalyst solution was fed into the space between outer surface of membrane and inner surface of reactor shell in the glove box. The actual feed side pressure was monitored and recorded each 20 sec with a pressure sensor on real-time at the retentate exit of the reactor. The liberated hydrogen in the permeance side of membrane was swept by constant flow rate Argon, with a typical volume flow rate of 200 mL/min, and the gas mixture was sequenced analyzed by one on-line GC (Agilent 7890A with PLOT/Q column) per 4 minute for the on-line composition analysis which allowed the hydrogen release rate to be calculated. The Pd membrane was washed with acetone and methanol several times and then dried at 100° C. after each reaction. The leakage of membrane with 0.8 MPa N₂ was checked by soap bubble meter (range 0-10 ml) before each reaction.

In a typical reaction, 1 mL of EtOH and PNN—Ru(II) catalyst mixture was loaded into the chamber. The reaction was then monitored until completion. The hydrogen permeance and the membrane surface area in the design ensure the capability of the Pd—Ag membrane in fully removing the produced hydrogen during the course of the reaction. FIG. 6 shows the hydrogen release rate and the total pressure of the chamber at time interval of 20 sec during the reaction when the reaction was carried out at temperature of 160° C. The final conversion of EtOH and the yield of EA were analyzed by NMR and/or GC-MS. Moreover, it was also confirmed with the total pressure inside the reaction chamber as well as the total volume of the hydrogen released by integrating the hydrogen release curve in FIG. 6. Based on the assumption that there is no hydrogen accumulation in the system, the total pressure inside the reaction chamber will be determined only by the partial pressures of EtOH and EA according to the Raoult's law where x is the molar ratio of EtOH in the reaction mixture. See Ref. 21. P=P _(EtOH) ^(Sat) x+P _(EA) ^(Sat)(1−x)

Take the reaction at 160° C. for example, based on the NMR and GC-MS analyses, the EtOH conversion was ˜99% and the EA yield was ˜99% after 24 hours. The EtOH conversion calculated from the hydrogen release curve was 96%, while the total pressure indicated that the EtOH conversion was 98%. The influences of reaction temperatures and catalyst loadings on the conversion of EtOH were examined. The results are summarized in Table 2. The conversions of EtOH with 0.02 mol % of catalyst were found to increase with elevated temperatures, reaching 36, 57 and 65% at 160, 180 and 200° C., respectively (FIG. 7). Increasing the catalyst loading at 160° C. improved the conversion from 36% (0.02 mol %) to 96% (0.10 mol %) and eventually to 99% (0.50 mol %) (FIG. 7). As can be seen, conversions calculated from different means agree very well with one another, indicating that the Pd—Ag alloy membrane can timely remove hydrogen from the system, and more importantly, suggesting that in the membrane reactor system the progress of the reaction can be monitored. This may potentially serve as a powerful tool to study the reaction kinetics.

The mechanical strength of the Pd—Ag membrane was proved by the tolerability to high pressure of 10-30 bar during the temperature range of 150-200° C. The membrane also showed good performances in thermal stability and chemical resistance over more than 500 hours of service for this reaction system. H₂ and N₂ flux of the membrane was measured between 150-350° C. after all reactions were finished. No N₂ leakage of the membrane was detected at room temperature with ΔP=800 kPa. However, the H₂ flux of Pd—Ag membrane at 350° C. with ΔP=100 kPa was 0.17 mol m⁻² s⁻¹, ca. 37% of the fresh membrane. At the same time, the activation energy of H₂ permeation was increased from 5.8 to 17.4 kJ/mol. The observations of the decline of H₂ flux and increase of permeance resistance were believed to be caused by the physical adsorption of reagents and catalyst on the surface of the membrane. The performance can be recovered to 0.37 mol m⁻² s⁻¹ (77% of the fresh membrane with the activation energy of 8.3 kJ/mol) after being washed with methanol and annealed in H₂ at 500° C. for 48 hours (FIG. 3). The morphology measurement of the Pd—Ag membrane after reaction also showed no coking formed on the surface (FIG. 4). While EtOH may decompose to CH₄, CO and H₂ in the presence of Pd catalysts,¹⁰ such reactions were not observed in the system as the inner pressure of the reactor was synchronously dropped to 0 when it was cooled to room temperature, indicative of no formation of other gases during the process.

TABLE 2 Dehydrogenative coupling of EtOH to EA and H₂ with PNN—Ru(II) complex 1 in a Pd—Ag membrane reactor. mol %^(a) T Conv. Yield Conv. Entry (×100) (° C.) (%)^(b) (%)^(b) V_(H2) ^(c) ΔP^(d) 1 1 200 25 98 22 27 2 2 160 36 98 33 40 3 2 180 57 99 59 60 4 2 200 65 99 66 68 5 10 160 96 99 94 93 6 50 160 99 99 96 98 ^(a)PNN—Ru(II) complex 1. ^(b)Determined by ¹H NMR and GC/MS. ^(c)Determined by H₂ produced. ^(d)Determined by the total pressure inside the reaction chamber.

In summary, that quantitative production of ethyl acetate and hydrogen directly from EtOH can be achieved in an ultrathin Pd—Ag/ceramic membrane reactor. This environmentally benign reaction is homogeneously catalyzed by dearomatized pincer Ru complexes, and no acid or base promoters and hydrogen acceptors are required. Defect-free Pd—Ag/ceramic membrane was employed for the in situ removal of the liberated hydrogen, allowing the reaction to be operated at elevated temperatures and reach almost 100% conversion and EA production yields. Furthermore, the liberated H₂ fluxes and the inner pressure of the reactor both can be used to accurately monitor the reaction progress in real-time.

REFERENCES

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Other embodiments are within the scope of the following claims. 

What is claimed is:
 1. A system for producing a dehydrogenative coupling product comprising: a reactor including a reactor shell and a selectively permeable tubular membrane placed inside the reactor shell so that a reaction zone is defined between an outer surface of the selectively permeable tubular membrane and an inner surface of the reactor shell, and a gas release zone is defined by an inner surface of the selectively permeable tubular membrane; a sweep gas line connected to the gas release zone for removing hydrogen from the gas release zone, and a catalyst selected from the group consisting of:

wherein the selectively permeable tubular membrane permits the hydrogen to pass through the membrane and substantially blocks a substrate and its dehydrogenative coupling product from passing through the membrane, wherein the substrate comprises a C1-C16 primary alcohol, wherein the selectively permeable tubular membrane includes a metal membrane formed on a solid support, the metal membrane including palladium and silver, and the solid support including aluminum oxide.
 2. The system of claim 1, wherein the gas release zone is configured to transport hydrogen out of the reactor.
 3. The system of claim 1, wherein the reaction zone surrounds the gas release zone.
 4. The system of claim 1, wherein the selectively permeable tubular membrane is cylindrical.
 5. The system of claim 1, wherein a thickness of the metal membrane is 5 μm.
 6. The system of claim 1, wherein the substrate and the catalyst are substantially free of solvent.
 7. A system for producing a dehydrogenative coupling product comprising: a reactor including a reactor shell and a selectively permeable tubular membrane, wherein a reaction zone is defined between an outer surface of the selectively permeable tubular membrane and an inner surface of the reactor shell, wherein a gas release zone, which is separated by the selectively permeable tubular membrane from the reaction zone, is defined by an inner surface of the selectively permeable tubular membrane, wherein the selectively permeable tubular membrane permits hydrogen to pass through the membrane and substantially blocks a substrate and its dehydrogenative coupling product from passing through the membrane, wherein the selectively permeable membrane includes a metal membrane formed on a solid support, the metal membrane including palladium and silver, and the solid support including aluminum oxide; a sweep gas line connected to the gas release zone, defined by the selectively permeable tubular membrane, for removing hydrogen from the gas release zone; and a catalyst selected from the group consisting of:


8. The system of claim 7, wherein the gas release zone is configured to transport hydrogen out of the reactor.
 9. The system of claim 7, wherein the reaction zone surrounds the gas release zone.
 10. The system of claim 7, wherein the selectively permeable tubular membrane is cylindrical.
 11. The system of claim 7, wherein a thickness of the metal membrane is 5 μm.
 12. The system of claim 7, wherein the substrate and the catalyst are substantially free of solvent. 